Ultrasound penetrates well through soft tissues and, due to its short wavelengths, can be focused to spots with dimensions of a few millimeters. As a consequence of these properties, ultrasound can and has been used for a variety of diagnostic and therapeutic medical purposes, including ultrasound imaging and noninvasive surgery. For example, focused ultrasound may be used to ablate diseased (e.g., cancerous) tissue without causing significant damage to surrounding healthy tissue. The noninvasive nature of ultrasound surgery is particularly appealing for the treatment of, for example, brain tumors.
An ultrasound focusing system generally utilizes an acoustic transducer surface, or an array of transducer surfaces, to generate an ultrasound beam. In transducer arrays, the individual surfaces are typically individually controllable, i.e., their vibration phases and/or amplitudes can be set independently of one another, allowing the beam to be steered in a desired direction and focused at a desired distance. In medical applications, the target location of the ultrasound focus is often determined using magnetic resonance imaging (MRI). In brief, MRI involves placing a subject, such as the patient, into a static magnetic field, thus aligning the spins of hydrogen nuclei in the tissue, and then applying radio-frequency electromagnetic pulses to temporarily destroy the alignment, inducing a response signal. Different tissues produce different response signals, resulting in a contrast among theses tissues in MR images. Thus, MRI may be used to visualize, for example, a brain tumor, and determine its location relative to the patient's skull. An ultrasound transducer system, such as an array of transducers attached to a housing, may then be placed on the patient's head, and the transducers driven so as to focus ultrasound onto the tumor. This method is referred to as magnetic-resonance-guided focusing of ultrasound (MRgFUS).
In MRgFUS, the treatment target is defined in magnetic resonance (MR) coordinates. To enable directing the ultrasound focus onto this target, the location and orientation of the transducer(s) need to be ascertained in MR coordinates as well. The transducer coordinates may be measured directly in the MR coordinate system using MR trackers—e.g., fiducials visible in MR images—that are rigidly attached to the transducer system, or have an otherwise fixed and known relative location with respect to the transducer(s). MR trackers may be implemented in various ways, for example, as MRI markers or microcoils.
Ideally, the acoustic surface and the MR trackers would be perfectly placed and aligned with respect to each other. In practice, however, mechanical tolerances in production are inevitable, and the relative positions of the transducer(s) and the MR trackers generally deviate from the nominal relative positions due to these “production errors.” As a result, if a transducer array is driven based on the nominal relative positions, the ultrasound focus will deviate from the intended focus. To ensure that the ultrasound focus more accurately coincides with the intended target, there is, accordingly, a need to quantify the effect of production errors on the accuracy of targeting.